1. Field of the Invention
This invention relates to an air-fuel ratio control system for an internal combustion engine having an exhaust gas recirculation (EGR) system, and more particularly to an air-fuel ratio control system of this kind, which is adapted to correct the air-fuel (A/F) ratio of a mixture supplied to the engine while recirculation of exhaust gases (EGR) is carried out by the exhaust gas recirculation system.
2. Prior Art
Conventionally, an air-fuel ratio control system has been proposed by the present assignee in Japanese Provisional Patent Publication (Kokai) No. 5-187288 (corresponding to U.S. Pat. No. 5,261,370), which is adapted to correct the amount of fuel to be supplied to the engine by a correction amount for use in the air-fuel ratio control, which is determined based on the engine rotational speed and load on the engine, while recirculation of exhaust gases is carried out by an exhaust gas recirculation system.
According to the proposed conventional system, the amount of fuel to be supplied to the engine is corrected by an amount of exhaust gases passed through a recirculation control valve of the exhaust gas recirculation system, which amount is estimated based on the correction amount for use in the air-fuel ratio control. The system contemplates response delay of the recirculation control valve and dynamic characteristics of recirculation gas. However, the amount of recirculation gas is not determined based on a detected value of valve opening of the recirculation control valve. As a result, the corrected fuel supply amount can cause undesired variations in the air-fuel ratio due to variations in operating characteristics between individual exhaust gas recirculation control valves to be used or aging of one used in the system, resulting in degraded exhaust emission characteristics.
Further, according to the proposed system, the valve opening of the recirculation control valve, is controlled based on the aforementioned correction amount. However, the response delay of the recirculation control valve and the dynamic characteristics of recirculation gas are not taken into account in controlling the valve opening, which also causes variations in the air-fuel ratio, resulting in degraded emission characteristics.